Automatic frequency control



June 2, 1953 c. wEN-rvwoRTH ET AL 2,640,918

AUTOMATIC FREQUENCY CONTROL Filed March 9, 1950 5 .wz\ u g ,s lr

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w \J"'* n S \l m- Y l* g Q *C @o o /NvE/vro/'s band/e1' /Vlltwaz'z liv-aleje A ORNEY Patented June 2, 1953 AUTOMATIC FREQUENCY CONTROL Chandler Wentworth and Hugh L. Donley,

Princeton, N. J., assignors to Radio Corporation of America, a corporation of Delaware Application March 9, 1950, Serial No. 148,694

9 Claims.

This invention relates to automatic frequency control (AFC) devices, and more particularly to thermal AFC devices which may be used in frequency modulation (FM) radio receivers or in the audio or sound channels of television (TV) receivers.

The present invention constitutes an improvement over the device disclosed in the copending Donley et al. application, Serial No. 146,166, filed February 25, 1950.

A thermal AFC device according to the aforesaid application (and also according to the present invention) comprises a high-temperaturecoeicient-of-capacitance 4titanate dielectric in intimate thermal contact with a very small high resistance semi-conductive heater element. The rst audio stage of a receiver can be used both as a control tube for the AFC unit and as an audio amplifier, the high resistance heater being used as the audio stage load resistance, The capacitance of the dielectric element rapidly follows the changes in temperature produced by the flow of direct current through the small heater element. The heater being in the first audio stage, the heater current is controlled by the D. C. component of the output of the discriminator of a FM or TV receiver; this current is therefore proportional to the frequency shift or drift of the local heterodyne oscillator. This frequency shift is automatically corrected through the change in capacitance of the temperature-responsive capacitor, such capacitance being coupled to the local oscillator circuit.

For use in a FM receiver, it is desirable to have the thermal AFC unit suiciently slow-acting to permit the receiver to be manually tuned in a normal manner, after which the AFC action would correct any mistuning or subsequent oscillator drift. To accomplish this purpose required a thermal AFC unit which took approximately l minute to warm up to its operating temperature after the receiver was turned on. The warm-up time of the receiver itself, however, was only about -20 seconds.

One difliculty encountered when the warm-up time of the AFC unit exceeds the warm-np time of the receiver itself is that the oscillator frequency may be at one extreme of its control characteristic (corresponding to a relatively low temperature of the control capacitor) when a signal is first received, and if a signal is present at this time the resulting AFC action holds it in this undesirable position, Another manifestation of the same effect may be noticed when the FM receiver is turned on a minute or two after (Cl. Z50- 20) 2 it has been turned on. while properly tuned to one of two closely spaced signals. When turned on, the receiver may select the wrong signal.

A solution to the aforementioned difficulty is that disclosed in the aforementioned application, viz., an auxiliary heater energized from the power line when the receiver is turned off or deenergized; such an auxiliary heater is in thermal contact with the dielectric element and maintains such element at substantially its operating temperature when the receiver is deenergized. The 11G-volt heating supply is removed just prior to the energizing of the regular or main heater; by using this scheme, the normal receiver warm-up time of about 15 seconds can be maintained.

An object of this invention is to devise a thermal AFC unit provided with an auxiliary heater and having in addition a shielding arrangement for preventing -cycley hum from being picked up by the main heater or the oscillator control capacitor, this shielding arrangement being provided without adding capacitance to the oscillator control capacitor. For best sensitivity for high frequencies, such capacitance should be kept small.

Another object is to provide a thermal AFC unit which not only serves for AFC purposes but also provides a radio frequency (RF) bypass capacitor for an audio stage. Such unit also serves as the load resistor for the first audio stage.

The foregoing and other objects and advantages of the inventionwill be best understood from the following description of an exemplification thereof, reference being had to the 4accompanying drawing, wherein the single gure is a part structural, part diagrammatic View of an AFC unit according to this invention as applied to a FM receiver of more or less conventional arrangement.

The objects of this invention are accomplished, briefly, in the following manner: A main heater and also an auxiliary heater are placed in thermal contact with a dielectric element composed of titanate material. lThe auxiliary heater is arranged to heat the dielectric element during periods of deenergization of the FM receiver in which the element is being used, to thereby maintain such element at substantially its operating temperature during such periods; the main heater is responsive to the output ot the discriminator to vary the temperature of the element to provide AFC during periods of energization of the receiver. A grounded metallic shield is provided between the auxiliary heater and the control the local oscillator frequency of the FMv In accordance with the disclosure inY receiver. the aforesaid copending application, element I is composed of a mixture of barium and strontium titanates which will provideior such element a very high temperature-coencient-of-ca pacitance as well as a high dielectricA factor, the composition of such mixture being varied to give the desired predetermined Curie temperatureor Curie point. As a typical example, element I may be composed of 70% barium Vtitanate and 30% strontium titanate. -Itis. element I which is utilized for AFC of thel local oscillator in the FM receiver, the capacitance ofl this element changing due to changes inthe temperature thereof and the capacitance of such element being coupled directly. intothe tank circuit of the local oscillator. The dimensionsr of element I lare quite small; as an. example, this element may have a diameter of 60 mils and a length of 1 3.; inch.

Positioned coaxially with element I isla main heater element 2, illustrated for convenience as being of substantially cylindrical connguration,

this heater element being in intimate thermal contact with element I and serving as the controllable heating means (controlled by the D. C. component of the output of the FM receivers discriminator) which varies the tempera-ture of element I in dependence upon'frequency changes inthe frequency being controlled, which is the input to the discriminator. In accordance with the disclosure in the aforesaid copending application, heater elementy 2 may be composed of a magnetite-barium strontium titanate mixture which will provide the desired high resistance per unit volume so that this heater can be made the plate load resistor of a vacuum tube, the composition or" this mixture being varied to give the desired predetermined resistivity. As a typical example, element 2 may be composed of y30% magnetite and 70% r(Il/3,0 barium strontium titanate. The dimensions of heater element 2 are also quite small; as example, ythis element may have a diameter of 40 mils and a length of 40 mils.

Element I is provided with fired-.on silver electrodes 3 and5, onelof which ,coversfeachcir'cnlar end face of such"eler'nent.` These electrodes may be obtainedI by ring at an elevated temperature a silver paste applied to suchfaces. In order to fasten. elements I and 2 together in intimate thermal Contact, heater element2k and capacitive element I are sweated together endto-end at electrode 3' with a small. amount ot" and may, for example, have a diameter of 60 mils and a length of 3U mils. The ond faces of Glenient 26 provided with firedmn :silver electrodes 4 and 35, heater element and clement EG being sweated together end-to-end at c tlodc 4 with soft solder. Electrode 35 is grour. ed to provide the shield hereinafter re-erred to.

Positioned coaxially with eleiur-n's 23, 2 etc. so far described at the end of ele: 12.6 opposite to element 2, is a dielectric element 35, illustrated foi-,convenience as being of substantially cylindrical configuration. which serves an ins-ula.` tor, as will later be described. Element 35 can be composed of the same tite-nate material as elements I and 26. insulatingT element 36 is also of small dimensions and may, :for example, be of the same dimensions element vifdiameter 60 mils and length 30 mils. Diele c elements 25 and 3 are sweated together cnr -to-cnd elect-rode S5, and at its other enf` element 3S is provided with a redon silver electrode 33.

Positioned coaxially with the elements so .lar described, at the end oi .cnient 3G opposite to element 25, is an Vauxiliary hea element 3I. illustrated for convenience as being of substan tially cylindrical conguration, this heater element being fastened in intimate tl mal contact with element 36 by being sweatcd to the end of element 3,6 at electrode 35i, Heater element 3| can be composed of the same magnetite-barium strontium titanate mixture asis heater 2 and functions to maintain capacitive eler'ient I o' s astantially its operating. temperature when the FM receiver is deenergiz'orl.` klement 3! may have the same dimensions as element viz., diameter 4() mils and len .h 4l) mils. At its exposed end, heater element 3| is provided with a fired-on silver' electrode 32 which covers its circular end face.

The time constant of any thermal device depends on its thermal capacitg7 or roughly on the mass of the device. For use in a receive it is desirable tohavetho AFC unit suf-noie f slow-acting to permit the receiver to be "I ally tuned in a normal manner, after whi AFC action would correct any mistuning or subsequent oscillator drift. lAlthoug n.` capccitufc element I having the dimensions given as an cie ample is suficienth7 slow-acting to permit the FM receiver to be manually tuned in a normal 1'1. shion, the warm-up timel of the AFC unit i f is about one minute. The dimensions of the dielectric element I and heater element 2 previously stated were found to give about the right mass of unit to provide a. control time (the time rc- Uquired for the AFC to retune the receiveri o vv gized when switch I is closed. The receiver includes in cascade a R. F., amplifier 8, a converter' stage 9 to which is supplied heterodyning energy from a local oscillator II), I. F. amplierII and a discriminator or ratio detector I2 which has a pair of output leads I3 and I4. lead I4 being grounded.` When the receiver is in operation,

the discriminator I2 will, at any instant of time, supply voltages across leads I3 and I4 which include a D. C. component. This component, un-

.. lless it is of zeromagnitude,4 will have some finite positive or negative value. The magnitude and polarity of this component are responsive to changes in frequency, from a predetermined value, of the mean signal frequency fed to discriminatori' I2.

Across leads I3 and I4 will also appear, as is well-known to those skilled in the art, audio frequency modulating voltages. The audio fre quency voltages are applied over condenser I5, volume control potentiometer I6 and condenser I'I to the grid I8 of the rst audio stage tube I9 the cathode 20 of which is grounded through resister 2 I. The D. C. component of the discriminator output voltage is applied through resistors 22 and 23 to control grid I8 of tube I9, which is thus used as a D. C. amplifier or control tube, the resistors 22 and 23 being connected in series between output lead I3 and grid It; an audio bypass condenser 24 is connected from ground to a point between resistors 22 and 23. Plate 25 of tube I9 is connected to main heater electrode 4 of heater 2, while heater electrode 3 is connected to the B supply; in this way, the plate load resistance or output resistance of tube I9 is the main heater 2 of the AFC unit.

Since electrode 4 is connected to plate 25 and electrode 35 is connected to ground, dielectric section 26 is connected from plate to ground of the rst audio stage IS. This section therefore functions as the RF bypass capacitor which is normally present (and required) at the plate of the rst audio stage. iiith the dimensions of element 26 previously stated, the capacitance of this capacitor is 20D-300 mmfd., a value such as usually used. It will be noted that the additional capacitor 26 is provided without the addition of capacitance to the main (thermal) capacitor I. Due to the fact that capacitor 26 is made of a material having a very large dielectric constant, suicient capacitance may be obtained with a capacitor of very small size.

The ampliiied audio frequency voltage is coupled through condenser 21 to the grid of the following audio stage.

Fixed bias is applied to the cathode 20 by means of a bleeder resistor 28 from the positive supply lead in combination with resistor 2|. a R. F. bypass condenser 29 being connected from such positive lead to ground. Resistor 28 is used to set the initial or operating bias on tube I9 such that the initial plate current through the main heater 2 (when the receiver is energized) maintains the operating temperature of the capacitor I well above ambient temperature when there is a zero or correct tuning point control voltage out of discriminator I 2, thereby eliminating the necessity for careful thermal shielding of the element.

The capacitance element i connected to the tank circuit of oscillator I by a lead extending from electrode through a series capacitor 3% to the plate of the tube in oscillator I9. By virtue of the R. F. bypass condenser 29 connected to electrode 3, the capacitance of the element I between the opposite electrodes 3 and 5 appears in eiect between electrode 5 and ground.

Assume that when the FM receiver is rst energized by closing switch l, the capacitor I is essentially or substantially at its operating temperature; that this is a proper assumption will hereinafter appear. The titanate capacitorheater unit I, 2 is then ready to function as a frequency control for the oscillator I@ when the receiver is first operative, which may be within 15 seconds after the receiver was turned on if such receiver has a warm-up time of 15 seconds.

The tube capacitances and circuit constants of the oscillator I0 will continue to change for some time after the receiver is turned on, to thereby cause the D. C. component of the discriminator output voltage to vary from zero. If the discriminator output voltage polarity with regard to frequency change has been correctly predetermined, this change in discriminator output voltage will cause the temperature of the titanate capacitor I to change in the proper direction to counteract the change in other circuit componente of the oscillator I0. For example, if the discriminator polarity is such that an increase in the mean I. F. supplied thereto applies a negative voltage to grid I8 to reduce the heater 2 current, the temperature of the capacitor I will de crease and the capacitanceoi such capacitor will increase, thus decreasing the frequency or the local oscillator I0; this change is in the correct direction to counteract the original slow change in mean I. F.

Because of the control time or time constant of the AFC unit (l0-15 seconds) the manual tuning process is similar to one with no AFC; thus, the operator can immediately tune in a station and the AFC action will compensate for tendencies of the oscillator I0 to drift. If when the receiver is first operative the tuning point is nearly correct for a particular station, the AFC action will correct the tuning of the oscillator in the proper direction to tune to the station.

As hereinbefore set forth and as described in the aforementioned copendingr application, certain difliculties which normally ensue, when the warm-up time of the AFC capacitor exceeds the warm-up time of the FM receiver, are solved satisfactorily by the provision of an auxiliary heater, in thermal contact with the capacitor I, which maintains such capacitor substantially at its operating temperature when the receiver is denergized. Such an auxiliary heater is denoted by numeral 3I and has previously been described.

In order to energize the auxiliary heater 3| in such a way that the heater draws a small amount of current from the 11G-volt A. C. power line when the power is ofi.r in the receiver, thereby to maintain element I at its operating temperature while the receiver is off, a lead extends from one side of power switch 'I to auxiliary heater electrode 33 and a lead extends from the other side of switch I (through a series resistor 34 if desired, to apply only an adjustable portion of the volts to heater 3|) to auxiliary heater elec trode 32. Instead of a series resistor 3A, a series capacitor can be used if desired.

When the receiver is on (switch 'I open) auxiliary heater 3l is energized from the ll0 volt line to maintain element I substantially at its operating temperature. When the receiver is energized, the on-oi switch I is closed, shunting out the auxiliary heater 3| and thereby removing the 11G-volt supply from such heater just prior to the energization of the regular or main heater 2. Consequently, when the power switch 1 of the receiver is closed, the AFC capacitor I is already at operating temperature while the current through the main heater 2 is rising to its predetermined operating value. The use of the auxiliary heater 3| therefore eliminates the time delay which would otherwise occur in the control capacitors reaching its operating temperature, so that as soon as the normal warm-up time of the receiver has passed (this warm-up time being, for example, 15 secfrequency.

as well as the main heater 2, the heat produced i by the auxiliary heater 3l is conducted to the control capacitor i at a slower rate than is heat produced by the main heater In fact, the auxiliary heater 3l requires about three times as much current as does the main heater 2 to get the capacitor element l to the same operating temperature. However, since the dimensions and power requirements of the whole AFC unit are very small, the above objection is of little concern, because the auxiliary heater merely provides enough heat (derived from power taken from the line), when the power switch l is open, to maintain the control capacitor i, as

well as the main heater E, at the desired operattemperature. in spite of the greater auxiliaryheater current requirements, only about 0.2 watt was needed from the power line to main-l tain the AFC unit in stand-by condition. Although there is a closed series circuit from the power line to power supply 6 through auxiliary heater 3i when switch l' is open, this is a very high resistance circuit and the amount oi current'lowing therethrough (e. g., 2 ma. es com pared to at least 590 ma. when switch 1 is closed to energize the receiver) is entirely insuiicient to produce any energization of the re'ctiiiers in power supply E at this time.

The presence of Gil-cycle voltage at the terminals of auxiliary heater El (and particularly at electrode 33, even when switch l is closed) requires shielding this auxiliary heater coth from the main heater 2 (which serves as the nrst audio stage plate resistor) and from the oscillator control capacitor i. Electrode 35, being grounded to the receiver chassis, acts as a shield to prevent SO-cycle hum from appearing across the main heater or across the control capacitor l. Locating the shield 35 shown shields the auxiliary heater 3i from the audio portion 2 and from the R. portion i of the unit without adding capacitance to the capacitor section i which controls the oscillator frequency. For

best sensitivity at high frequency, this capacitance should he kept small.

As previously stated, locating the shield as illustrated provides dielectric section 26 which serves as a R. F. bypass condenser for the first audio stage, The second dielectric section 36 is necessary because the power line, which energizes the auxiliary heater 3i, does not normally have one side directly connected to the chassis ground.

To summarize, the AFC unit of this invention not only serves for AFC purposes, but also as the first audio stage load resistor and R. F. bypass. The addition oi the small high resistance auxiliary heater 3i consuming a negligible amount of power from the A. C. power line makes possible a rapid warm-up time FM receiver with AFC action which causes no confusion to the person tuning the receiver. It will be appreciated that the AFC unit is shown greatly exaggerated 'in size in the drawing, for purposes of clarity.

Whatwefclaim to be our invention is:

l. In a radio receiver, signal-responsive means for producing an output voltage in response to a slow change, from a predetermined frequency, in thefrequency of the signal fed thereto, a controllahle-frequency generator in said receiver for varying the frequency of the signal fed to said means, a capacitive element the capacitance of which varies with `the temperature thereof, a main electrical heater element in intimate thermal contact with said capacitive element, means responsive to said produced .f'oltage for controlling the current through said heater element, means coupling capac.' ive element to said generator. the arrangement being such that the frequency oi said generator is controlled,

thereby varying the frequency of the signal fed to said signal-responsive means, in response to variations in capacitance of said capacitive ele-V ment, an aux? ary heater element spaced from said main hea r element and in intimate ther-Y mal Contact with Said capacitive element, means for causing encrgiaation of said auviliary heater element in responso 'to dcencrgization of said receiver, a dielectric element interposed between said niain and auxiliary heater elements,l

and means for uti ne' the capacitance provided by said dielectric element as a radio ircquency bypass capacitor in said receiver.

2. In a radio receiver adapted to be energized at will from a source oi power through a switch, signal-responsive means operative in response to closing of said .f-:itch to produce an output voltage in response to a slow change, from a predeterinne frequency, in the frequency of the sigfed thereto, a controllable-frequency generator in said receiver for varying the frequency of the signalfed to said means, a capacitive ele-V ment the capacitance of which varies with the temperature thereof, a main electrical heater element in intimate thermal contact with said ca'- pac'tive element, means responsive to said produced voltage for controlling the current through said heater element, means coupling said capacitive element to said generator, the arrangement being such that the frequency of said generator is controlled, thereby varying the frequency of the signal fed to said signal-responsive means, in response to variations in capacitance of said capacitive element, an auxiliary electrical heater element spaced from said main heater element and in intimate thermal Contact with said capacitive element, means connecting said auxiliary heater element across said switch, a dielectric elcment interposed between said main and auxiliary heater elements, and means for utilizing the capacitance provided by said dielectric element as a radio frequency bypass capacitor in said receirer.

In a radio receiver adapted to be energized at will from a source of power through switch, signal-responsive means operative in response to closing of said switch to provide an output voltage in response to a slow change, from a predetermined frequency, in the frequency of the signal led thereto, a controllable-frequency generator in said receiver for varying the frequency of the signal fed to said means, a capacitive element the capacitance of which varies with the temperature thereof, a :nain electrical heater element in intimate thermal Contact with said capacitive element, means responsive t0 said produced voltage for controlling the current through said heater element, means coupling said capacitive element to said'generator, the arrangement being such that the frequency of said generator is controlled, thereby Varying the frequency of the signal fed to said signal-responsive means, in response to variations in capacitance of said capacitive element, an auxiliary electrical heater element spaced from said main heater element and in intimate thermal contact with said capacitive element, means connecting said auxiliary heater element across said switch, a metallic shield connected to a point of aero reference potential, said shield having said auxiliary heater element on one side thereof and said main heater element and said capacitive element on 'the other side thereof, a dielectric element interposed between said main and auxiliary heater elements, and means for utilizing the capacitance provided by said dielectric element as a radio frequency bypass capacitor in said receiver,

4. In a radio receiver adapted to be energized at will from a source of power through a switch, signal-responsive means operative in response to closing of switch to provide an output volt-- age in response to a slow change, from a predetermined frequency, in the frequency of the signal fed thereto, a controllable-frequency gcnerator in said receiver for varying the frequency oi the signal fed to said means, a capacitive elev ment the capacitance of which varies with the temperature thereof, a main electrical heater elcment in intimate thermal contact with said cap citive element, means responsive to said produced voltage for controlling the current through said heater element, means coupling said capacitive element to said generator, the arrangement being such that the frequency of said generator is controlled, thereby varying the frequency of the signal fed to said signal-responsive means, in response to variations in capacitance of said capacitive element, an auxiliary electrical heater element spaced from said main heater element and in intimate thermal contact with said capacitive element, means connecting said auxiliary heater element across said switch, a metallic shield connected to a point of zero reference potential, said shield having said auxiliary heater element on one side thereof and said main heatelelnent and said capacitive element on the other side thereof, an insuiating element interposed between said auxiliary heater element and shield, a dielectric element interposed between said shield and said main heater element, and means for utilizing the capacitance provided by said dielectric element as a radio frequency bypass capacitor in said receiver.

In a radio receiver, signal-responsive means for producing a direct output voltage in response to a slow change, from a predetermined frequency, in the frequency of the signal fed thereto, a controllable-frequency generator in said receiver for varying the frequency of the signal fed to said means, a capacitive element the capacitance of which varies with the temperature thereof, a main electrical heater element in intimate thermal contact with said capacitive element, a direct current amplifier stage having an input circuit coupled to the output of said means and having an output circuit, means coupling said heater element to said output circuit as the output load resistor for said stage, to thereby control the temperature of said capacitive element in dependence upon the direct voltage output of said first-named means, means coupling said capacitive element to said generator, the arrangement being such that the frequency of said generator is controlled, thereby varying the frequency of the signal fed to said first-named means, in response to variations in capacitance of said capacitive element, an auxiliary heater element spaced from said main heater element and in intimate thermal contact with said capacitive element, a dielectric element interposed between said main and auxiliary heater elements, and means for utilizing the capacitance provided by said dielectric element as a radio frequency bypass capacitor in said ampliner stage.

6. In a radio receiver, signal-responsive means for producing a direct output voltage in response to a slow change, from a predetermined frequency, in the frequency of the signal fed thereto, a controllable-frequency generator in said receiver for varying the frequency of the signal fed to said means, a capacitive element the capacitance of which varies with the temperature thereof, a main electrical heater element in intimate thermal contact with said capacitive element, a direct current amplifier tube having a grid coupled to the output of said means and having a plate circuit, means coupling said heater element to said plate circuit as the plate load resistor for said tube, to thereby control the temperature of said capacitive element in dependence upon the direct voltage output of said first-named means, means coupling said capacitive element to said generator, the arrangement being such that the frequency of said generator is controlled, thereby varying the frequency of the signal fed to said first-named means, in response to variations in capacitance of said capacitive element, an auxiliary heater element spaced from said main heater element and in intimate thermal contact with said capacitive element, a dielectric element interposed between saidy main and auxiliary heater elements, and means for utilizing the capacitance provided by said dielectric element between the plate of said tube and a point of zero reference potential.

'7. In a radio receiver adapted to be energized at will from a source of power through a switch, signal-responsive means operative in response to closing of said switch to produce a direct output voltage in response to a slow change, from a predetermined frequency, in the frequency of the signal fed thereto, a controllable-frequency generator in said receiver for varying the frequency of the signal fed to said means, a capacitive element the capacitance of which varies with the temperature thereof, a main electrical heater element in intimate thermal contact Wit-h said capacitive element, a direct current amplifier stage having an input circuit coupled to the output of said means and having an output circuit, means coupling said heater element to said output circuit as the output load resistor for said stage, to thereby control the temperature of said capacitive element in dependence upon the direct voltage output of said iirst-named means, means coupling said capacitive element to said generator, the arrangement being such that the frequency of said generator is controlled, thereby varying the frequency of the signal fed to said first-named means, in response to variations in capacitance oi' said capacitive element, an auxiliary electrical heater element spaced from said main heater element and in intimate thermal contact with said capacitive element, means connecting said auxiliary heater element across said switch, a dielectric element interposed between said main' and auxiliary heater elements, and means for utilizing the capacitance provided by .to closingof said switch to produce a direct outn put voltage in response to a slow change, from a predetermined frequency, in the frequency of theV signal fed thereto, a controllable-frequency generator' in said receiver for varying the frequency ofthe `signal fed to said means, a capacitive element the `capacitance of which varies with j the temperature thereof, a main electrical heater element in intimate thermal contact with said capacitive element, a direct current amplifier stage having an input circuit coupled to the output of said means and having an output circuit, means coupling said heater element Yto said output circuit ,as the output load resistor for said stage, to thereby control the temperature of said capacitive element in dependence upon the direct voltage output of said first-named means, means couplingsaid capacitive element to said generator, -the arrangement beingsuch that the frequency of said generator is controlled, thereby ,varying the frequency of the signal fed to said Yfiirst-named means, in response to variations in capacitance of said capacitive element, an aux- "iliary electrical heater element spaced from said main heater element and in intimate thermal contact with Asaid capacitive element, means connecting said auxiliary heater element across said switch, a metallicl shield connected to a point of zero reference potential, said shield having said auxiliary heater element on one side thereof and said main heater element and said capacitive element on the other side thereof, a dielectric element interposed'between said main and auxiliary heater elements, and means for utilizing the capacitance provided by said dielectric element as a radio frequency bypass capacitor in said amplier stage.

19. In a radio receiver adapted to be energized at will from a source ofpower through a switch,

'signal-responsive means operative in response to 'closingof said switch to provide a direct output voltage in response to a slowchange, from apre- -deterrnined `frequency, iny the frequency of the signal fed thereto, a controllable-frequency gen- `erator in said receiver forvarying the frequency ofthe signal ed to saidmeans, a capacitive element the capacitance of which varies with the temperature thereof, a main electrical heater element in intimate thermal contact with .said capacitive element, a direct current amplierftube having a grid coupled to the output of said means and having a plate circuit, means coupling-said heater element to said'plate circuit as the plate load resistor for said tube, to thereby control the temperature of said capacitive element in dependence upon the direct'vclt-age output of said first-named means, means coupling said capacitive element to said generator, the arrangement being such that the frequency of said generator is controlled, thereby varying the frequency of the signal fed to said first-named means, in response to variations in capacitance of said capacitive element, an auxiliary electrical vheater element spaced from said main heater elementand in intimate thermal contact with said capacitive element, ineans connecting said auxiliary heater element across said switch, a metallic shield connected to a point of zero reference potential, said shield having said auxiliary heater element on one side thereof and said niain heater element and said capacitive element on the other side thereof, an insulating element joined to said auxiliary heater elementand said shield and interposed therebetween, dielectric element joined to said shield and said main heater element and interposed therebetween, and means connecting the main-heater end of said dielectric element to the plate ci said tube,` to thereby utilize the capacitance provided by said dielectric element between tiie plate of said tube and a point of zero reference potential, as a radio frequency bypass capacitor.

CHANDLER` VVENTWORTH. HUGH L. DONLEY.

References Cited in the le of'this patent UNITED STATES PATENTS Number Name Date 1,894,687 Hyland Jan. 17, 1933 2,126,027 Muth et al Aug. 9, 19,38 2,210,406 Henderson Aug. 6, 1940 2,483,070 Spindler Sept. 27, 1949 

